If you went to the supermarket yesterday, the National Security Agency may know all about it. Your cellphone may be among 5 billion that are monitored each day by the NSA, allowing the agency to track your movements along with the cellphone in your pocket or purse.

These are the latest revelations from The Washington Post, which has looked at documents provided by former NSA consultant Edward Snowden and interviewed government officials. It appears the NSA has a preposterously wide reach — so wide that ordinary citizens may throw up their hands and conclude: If I am among 5 billion, what can they really learn and what can it matter?

The immensity of the agency’s monitoring program raises the haystack problem. Critics say that intelligence agencies looking for the terrorist needle in the haystack of the world may be making their job all the more difficult by enlarging the haystack to such an extreme degree. Now the NSA is watching our cellphones on the drive to work or on the way to Grandma’s house for turkey dinner. Where does that get them?

Presumably, the intelligence agencies are developing methods of identifying contacts between cellphones of interest and other cellphones. We already know the agency has developed methods of tracking phone calls between a terrorist in Yemen and a would-be terrorist in Minneapolis. Now, apparently, they can follow the would-be terrorist to the airport as he prepares to embark for the Middle East.

Shortly after the first revelations about NSA spying, an acquaintance of ours emailed a friend, saying he had just returned from Pakistan, where he had learned to make bombs. It was a joke! It was meant as a taunt to would-be spies who might have been tracking references to Pakistan in private emails of American citizens. Come after me if you like — that was the message. They never did.

But there are a lot of people in America calling or emailing Pakistan and other trouble spots for reasons entirely innocent: talking to friends or relatives; arranging business trips; planning visits. If NSA spying is as pervasive as recent revelations suggest, their expectation of privacy has been sharply diminished, which represents a serious and continuing erosion of constitutional rights.

Supporters of the present surveillance regime say that if you’re not doing anything wrong, you have nothing to worry about. That is not the way our constitutional rights are supposed to work. It is offensive and oppressive for government authorities to rifle through an innocent citizen’s papers or to follow him or her on the street. Innocent people are supposed to be left alone. It is one of the blessings of democracy.

Five billion cellphones a day pushes the NSA’s monitoring efforts to an extreme that becomes almost laughable. The lives of most of us are too boring to make much of an impression on the NSA’s map of cellphone movements. We represent the vast haystack of human activity.

We know that within that vast haystack there are a few individuals who ought to be monitored, followed, spied on. It is the burden of law enforcement and intelligence agencies that they carry out their duties without compromising the rights of the vast majority of citizens. Sen. Patrick Leahy, chairman of the Senate Judiciary Committee, has said he intends to bring NSA activities within the purview of the Constitution.

It’s hard to know what to think of the vast program of surveillance that includes the monitoring of 5 billion cellphones. If an individual feels securely anonymous within that vast haystack, the fact of that Orwellian apparatus itself does not inspire a feeling of security. We understand that the nation is threatened by shadowy groups who mean to do us harm. But as the government carries out its duties to protect us from those threats, it is important that it not create new threats to our fundamental rights.